1. Field of the Invention
The present invention relates to a heat exchanger used in air-conditioning, refrigeration or the like and adapted to transmit heat between fluids such as a refigerant and air.
2. Prior Art
This type of heat exchanger illustrated schematically in FIG. 1 generally has copper tubes 1 connected to each other by means of U-bends as well as fins 2 made of aluminum and is arranged such that a refrigerant flowing through the copper tubes 1 and air 3 flowing through the fins 2 effect heat exchange. In recent years, there has been a demand for such types of heat exchanger to be made compact and to be provided with high performance. However, since the speed of air flow between adjacent fins is checked to a low level with a view to reducing noise and for other reasons, the air flowing outside the tube experiences a greater heat resistance than the refrigerant flowing inside the tube. Therefore, a common measure which is currently taken is to expand the air-side heat area, thereby reducing the difference in heat resistance between the air-side and refrigerant-side tube surfaces. However, an attempt to expand the heat area encounters certain physical limitations, and there are also problems in terms of economic efficiency, the need for saving space, etc. Therefore, the achievement of a reduction in air-side heat resistance in such a heat exchanger has been regarded as an important task. In addition, heat exchangers provided with slits, louvers or the like on the fin surfaces thereof have come into use in recent years. However, if such a heat exchanger is used for an outboard heat pump unit, frosting occurs during winter to a remarkable degree, so that it becomes necessary to effect defrosting more frequently than in the case of a heat exchanger employing flat fins. For this reason, a room temperature may change to a remarkable extent to the discomfort of occupants. In terms of economic efficiency as well, the seasonal efficiency in winter resulting from energy loss during the changeover to the defrosting mode suffers decline with these types of heat exchangers. Consequently, fins provided with slits, louvers or the like are employed for the inboard unit of a heat-pump cooling and heating machine, while flat fins are used for the heat unit of the outboard unit. In cases where flat fins are used, the outboard unit becomes large in size, resulting in increased production costs, so that there has been a demand for some improvement in this respect.
FIGS. 2a and 2b show an example of a conventional heat exchanger which does not have slits. FIG. 2a is a top plan view, while FIG. 2b is a side elevational view. A refrigerant such as Freon circulates through copper tubes 4, and the heat of the refrigerant is transmitted from the copper tubes 4 to fin collars 5 and then to fins 6. The air 7 flows backward from the front of the fins 6 and passes between the adjacent fins 6. At that time, heat is exchanged between the air coming into contact with the fins 6 and the surfaces of the fins 6 which have a different temperature and to which heat is transmitted from the refrigerant. Owing to this action, heat exchange between the refrigerant and the air is carried out continuously.
Such a fin is poor in terms of performance but excels in terms of frosting characteristics, namely, any decline in its performance resulting from frosting is low. For this reason, such fins are used for the outboard unit of a heat pump cooling and heating machine. However, if comparison is made between a heat exchanger constructed with such flat fins and a heat exchanger provided with slits, louvers or the like, the former has a disadvantage in that the weight or volume per unit capacity becomes extremely large since the performance thereof is poor. For this reason, various measures have been proposed to improve the frosting characteristics, but it has been difficult to find an arrangement of fins in which both the frosting characteristics and performance excel at the same time.
Meanwhile, FIGS. 3a and 3b show an example of a conventional heat exchanger having cutouts. FIG. 3a is a top plan view, while FIG. 3b is a cross sectional view taken along the line IIIa-IIIb of FIG. 3a. A refrigerant such as Freon circulates through copper tubes 5, and its heat is transmitted from the copper tubes 5 to fin collars 6, and then to fins 7 and cutouts 8. Meanwhile, the air sent in the direction of an arrow 9 by means of a fan passes between the adjacent fins 7, and, at that juncture, transmits heat to the fin surfaces having a different temperature or absorbs heat from the same.
The heat exchanger of this conventional type is called a slit-fin type having the cutouts 8 in each fin 7. If such a fin is compared experimentally with a flat fin which is not provided with slits or the like, the former exhibits surface heat resistance reduced by 40 to 50% compared with the latter. Theoretically, however, if cutouts are thus provided on the fin surface, since the heat transfer coefficient of the laminar flow in the entrance region becomes extremely high, it should be possible for the heat resistance coefficient of the fin surface to be lowered by 50% or more. The difference between this theoretical value and the experimental value can be accounted for by various factors, the major of which may be cited as follows: (1) the pressure loss of the air flow passing the cutouts 8 is higher than the other portion, so that the amount of air passing through the cutouts 8 declines, and sufficient use is thus not made of the thermal performance. (2) Since a large dead water region exists, the effective heat transfer area is reduced. Since the dead water region downstream of each copper tube 5 located on the upstream side of the air flow 9 covers the cutouts 8 located therebehind, the heat resistance of these cutouts 8 increases, thereby increasing the average heat resistance of the fins. (3) Since the copper tubes 5 are disposed in a staggered manner and the cutouts 8 are provided in front of or at the rear of the copper tubes 5, the heat flux from the copper tubes 5 is prevented, resulting in a decline in fin efficiency.
However, because a fin provided with slits has an adequate performance unlike a flat fin, heat exchangers having such an arrangement have been used as the inboard unit of a heat-pump cooling and heating machine, or as a unit exclusively used for cooling. The reason for this is that a fin having such slits has poor frosting characteristics despite the fact that its overall performance is high.
Through measures for rearranging the positional relationship between the fins and the heat transfer tubes, the present invention provides an arrangement of fins having cutout which has an unprecedentedly high performance and which experiences less frosting, as well as an arrangement of fins which has extremely good frosting characteristics and a high performance.